1. Field of the Invention
The present invention relates to a senseless startup control technique of a brushless direct current motor (hereinafter referred to as BLDC motor), and, more particularly, to a method that is capable of controlling a BLDC motor to prevent the generation of transient current in its startup stage.
2. Description of the Related Art
Generally, a BLDC motor includes a stator as an armature inducing electromotive force (emf) by a current-carrying coil thereof, and a rotor as a permanent magnet having N-S poles, in which the rotor is rotated with respect to the stator. It is necessary to form continuous rotating magnetic field of the BLDC motor such that the BLDC motor can be successively operated. Phase commutation of current flowing through each of windings is performed at a proper time to form a continuous rotating magnetic field, which can be achieved as a rotor position is correctly recognized. Here, the term “commutation” means to change the direction of current flowing through the windings of the stator such that the rotor can be rotated with respect to the stator.
To smoothly operate the BLDC motor, the rotor position is accurately consistent with the commutation time point of the phase current, which requires a device for detecting the rotor position. Although the device employs a position sensor, such as a Hall sensor, a Resolver sensor, and an encoder, the position sensor causes problems such as an increase of manufacturing cost of the BLDC motor, complication of driving circuits, etc. To resolve such problems, a method for detecting rotor position using electric circuits is researched.
Thus, an electric circuit for detecting rotor position is developed using back-emf of a BLDC motor. As such, the BLDC motor is operated by the rotor position that is detected by an electric circuit, instead of a position sensor. Hence, it said that “the motor is operated in a sensorless operation mode.”
Before a BLDC motor is operated in a senseless operation mode, it must pass through a startup stage, which will be described below with reference to FIGS. 1 and 2.
In FIG. 1, current is supplied to two phases of the BLDC motor (for example, if the BLDC motor has three phases) such that a rotor is forcibly aligned as illustrated in operation 1. At operation 2, after completing the alignment of the rotor, the amplitude and frequency of the voltage applied to the BLDC motor are varied such that the rotation speed of the rotor of the BLDC motor can be forcibly accelerated to reach a predetermined speed, as shown in FIG. 2, in operation 2. At operation 3, when the rotation speed of the rotor reaches the predetermined speed such that back-emf can be detected from the stator winding of the BLDC motor, the voltage supplied to the BLDC motor is controlled such that magnetic field phases of the rotor and the stator are adjusted. After that, at operation 4, the present operation mode of the BLDC motor is converted into a sensorless operation mode such that phase commutation and speed control of the BLDC motor are performed to run the BLDC motor, based on position information of the rotor, in which the position information is indirectly detected from back-emf information of the BLDC motor.
However, the conventional sensorless operation control method of the BLDC motor requires position information of the rotor to drive the BLDC motor. If a voltage is supplied to a stator winding in a state wherein position information of the rotor is not recognized, a transient current is generated. The transient current causes the generation of relatively large torque pulsation and the demagnetization of a permanent magnet of the rotor.
Also, since a voltage with a specific frequency pattern is applied to the BLDC motor regardless of the substantial position of the rotor before the BLDC motor is converted into the sensorless operation mode, the transient current causes the generation of torque pulsation and the demagnetization of a permanent magnet. Furthermore, the excessive torque pulsation may cause failure of conversion of the sensorless operation mode.
To resolve the above mentioned problems, U.S. Pat. No. 5,859,520 discloses that a lead voltage of the BLDC motor is detected to be compared with a voltage of ‘0,’ and a zero crossing point (ZCP) of back-emf thereof is detected. After that, the BLDC motor is delayed by a time corresponding to one-half of the average ZCP detection time of the back-emf to perform phase commutation.
According to a method disclosed in U.S. Pat. No. 5,859,520, only the switching elements of the inverter are controlled by a PWM, and a lead voltage of a phase to which a voltage is not applied is compared with a voltage of ‘0’ to detect a ZCP of back-emf. After that, the BLDC motor is delayed by a time corresponding to one-half of the average detection time point of the ZCP of the back-emf to perform phase commutation. Similar to the descriptions of FIGS. 1 and 2, the BLDC motor must be synchronously accelerated until a ZCP of the back-emf can be detected.
However, the conventional method has disadvantages in that, since the BLDC motor must be synchronously accelerated until the ZCP of the back-emf can be detected, it has a high possibility that a transient current can be generated. Also, the conventional method has drawbacks in that, when the BLDC motor is delayed by a time corresponding to one-half of the average detection time point of a ZCP of a back-emf to perform phase commutation, a system generating large load variation at a very low speed, such as a compressor, cannot perform phase commutation at a correct phase commutation time point. Therefore, in the conventional methods, there is a high possibility that a transient current can be generated.
Furthermore, IEEE (“Four-quadrant sensorless brushless ECM drive”, 10–15 Mar., 1991) discloses a startup control method of a senseless BLDC motor that is capable of integrating back-emf, comparing the integration result with a specific voltage level, and then performing phase commutation based on the comparison. However, since the conventional methods must synchronously accelerate the BLDC motor until a back-emf can be detected, there is a high possibility that a transient current can be generated at a startup of the BLDC motor.